Unlike liquid-crystal (LC) and light-emitting display (LED) technologies, non-emissive electrochromic (EC) systems benefit from their ability to be viewed from a wide range of angles under a wide range of ambient lighting conditions, such as direct sunlight. The potential for integration of EC systems into electrochromic devices (ECDs), such as: low-driving-voltage powered information panels and tags; smart windows and mirrors; and portable operating systems, including shape-conforming electronic papers, has promoted development of novel EC materials. The ability to print discrete electrochromic pixels can allow the combining of colors in portable display applications, such as information tags and electronic papers.
When compared to their inorganic counterparts (e.g. MoO3, NiOx), non-emissive organic electrochromics offer the potential for cost-effective, ambient atmosphere solution-processing over large areas and mechanically deformable surfaces. Viable organic EC materials for the development of commercially attractive ECDs must be synthetically accessible, have long-term redox stability, be processable from solution, and display good film-forming properties. The combination of these features is difficult to attain with small molecules and has motivated the use of πconjugated electroactive polymers (ECPs).
The utility of pi-conjugated polymers (CPs) for electrochromic applications was first suggested independently in Gazard et al., “Electrooptical Properties of Thin Films Of Polyheterocycles” Journal de Physique Colloques 1983, 44, 537-42 and Druy et al., Poly (2,2′-Bithiophene): an Electrochromic Conducting Polymer” Journal de Physique Colloques 1983, 44, 595-8, where redox switching of electropolymerized polythiophenes resulted in a color change. Inspection of their red-to-blue color-changing pattern on progressive electrochemical oxidation revealed a p-doping process governed by the bleaching of their pi-pi* transition in the visible with simultaneous appearance of infrared charge-carrier optical transitions tailing into the red region to induce a characteristic blue oxidized state. In general, charged carriers balanced with counter ions are produced along pi-conjugated organic polymer backbones subjected to increasing doping levels. With the introduction of charged carriers, namely radical cations (polarons) and dications (bipolarons), new optical transitions arise at longer wavelengths, and this process is accompanied by the simultaneous depletion of the ground-state optical absorption of the system being doped. The ability of the backbone to assume a stable quinoidal geometry influences the level of doping achievable, and, in turn, the extent of bleaching attained by the ground state absorption.
In addition to the synthetic accessibility of ECPs, the ability to achieve palettes of colors by changing the polymer's repeating unit structures, and the disposition of the structures along the chain, makes CP systems notable for development of processable EC materials. For this reason, extensive research efforts have been directed to tailoring the complex interplay between polymer structure and optical absorption in systems exhibiting important spectral changes in their successive redox states. Since the discovery of electrochromic effects in substituted and unsubstituted polythiophenes, a library of thiophene-, pyrrole-, and many other heterocycle-containing pi-conjugated electrochromic hybrids, which reflect or transmit distinct colors on electrochemical doping, have been developed that span all the useful colors for a display device. While multichromic polymers, those having different colored states when fully reduced or oxidized, may be useful in configurations where the attainable color states on redox switching match the color requirements specific to the application being considered, the ability to turn colors “on” and “off” is even more attractive. When “off”, the ECP has a transmissive redox state where all visible absorption of the chromophore is fully depleted with absorption in the near-IR allowing a device made thereof to transmit all visible colors, but when “on”, the ECP is in a redox state with a strong visible absorption. Cathodically-coloring ECPs switch from a colored neutral state to a transmissive state on electrochemical oxidation, while anodically-coloring ECPs switch from a transmissive neutral state, generally where the ground state absorption lies in the UV, to a colored oxidized state on doping, where the absorption lies within the visible spectrum. Ultimately, the extent of transmissivity of the “colorless” state depends on the position of the charge carrier transitions in a cathodically-coloring ECP, or on the position of the ground state absorption in the anodically-coloring ECP, relative to the visible spectrum.
In spite of the quantity of research directed to the synthesis and characterization of π-conjugated ECPs with desirable color states, examples of cathodically- and anodically-coloring polymers remains sparse when compared to multichromic ECPs. The most widely reported cathodically-coloring ECPs are poly(dioxythiophene)s such as poly(3,4-ethylenedioxythiophene)s (PEDOTs) and poly(3,4-propylenedioxythiophene)s (PProDOTs), which are easily oxidized from a neutral purple-blue-colored state to a highly transmissive doped state. The need to fine-tune colors has lead to a number of spectral engineering principles for ECPs. One approach to spectral control is the ‘donor-acceptor’ approach, where electron-rich and electron-deficient moieties alternate along a π-conjugated backbone. This approach has produced dual-band and broadly-absorbing chromophores that exhibit neutral color states that, generally, have not been attained by π-conjugated ECPs, for example blue-green, green and black colored states.
The promotion of ECPs to the forefront of organic electronics with commercial applicability requires a parallel development of sustainable solution-processing approaches that are low-cost and can be carried out under ambient conditions with non-toxic solvents and additives amenable to high throughput. While a number of π-conjugated electrochromic polymers (ECPs) having varying color states and redox switching properties have been developed, only a limited number of electrochromic polymers are print- or spray-processable from conventional organic solvents. In general, ECPs have used hydrocarbon or ethereal pendant groups to render the polymers soluble in organic solvents, requiring the use of flammable and environmentally hazardous solvents such as toluene, chloroform, or tetrahydrofuran for processing. Once processed, the ECP films must also be redox switched in high dielectric organic solvents, such as propylene carbonate or acetonitrile, and employ expensive organic-soluble electrolyte salts, such as lithium bis-trifluoromethanesulfonimide.
No attempt to print or spray thin films of ECPs from aqueous solution has been reported, likely due to problems of solubility and/or film-formability from a polar medium having a relatively low vapor pressure. The sole example of printing a conjugated polymer (CP) using an aqueous vehicle has been the printing of an aqueous emulsion of polystyrene sulfonate doped poly(ethylene-3,4-dioxythiophene) (PEDOT:PSS); however, most CPs do not disperse in water to a sufficient concentration to utilize a printing method. The first water soluble CPs were introduced in the late 1980's and polymers processable from aqueous solvents have been used in applications such as biochemical sensing, organic light emitting diodes, organic photovoltaics, and field effect transistors. In applications where multiple layers of different materials are needed, water soluble materials can be orthogonally deposited with materials that are soluble in and processed from organic solvents. In the vast majority of cases, the deposition method employed for the fabrication of solid state devices made with organics is spin-coating or layer-by-layer depositions when ionic molecular structures can be accessed. Neither method has shown to be commercially viable to date and high-throughput approaches remain undeveloped. Low-cost and high-throughput viable processing techniques based on environmentally benign aqueous solutions is desired for ECP and other CP films for use in devices, such as biochemical sensors, organic light emitting diodes, organic photovoltaics cells, field effect transistors, and electrochromic displays.